The present invention relates generally to a method for manufacturing a grid for gating a stream of charged particles.
Certain types of particle measurement instruments, such as ion mobility spectrometers, can require a gating device for turning on and off of a flowing stream of ions or other charged particles. This is accomplished by disposing a wire grid within the path of the ions; alternately energizing or de-energizing the grid then respectively traps the ions or allows them to flow.
Certain types of time of flight spectrometers, such as those described in the paper by Vlasak, P. R., et al., entitled “An interleaved comb ion deflection gate for m/z selection in time-of-flight mass spectrometery,” in Review of Scientific Instruments, Vol. 67, No. 1, January 1996, pp. 68–72, also utilize a gating device.
The most common methods for accomplishing this use an interleaved comb of wires also referred to as a Bradbury-Nielson Gate. Such a gate consists of two electrically isolated sets of equally spaced wires that lie in the same plane and alternate in potential. When a zero potential is applied to the wires relative to the energy of the charged particles, the trajectory of the charged particle beam is not deflected by the gate. To deflect the beam, bias potentials of equal magnitude and opposite polarity are applied to the two sets of wires. This deflection produces two separate beams, each of whose intensity maximum makes an angle alpha with respect to the path of the un-deflected beam.
One approach to manufacturing a gating grid is disclosed in U.S. Pat. No. 4,150,319 issued to Nowak, et al. In this technique, a ring-shaped frame is fabricated from a ceramic or other suitable high temperature material. The two sets of wires are wound or laced on the frame. Each set of wires is actually a single, continuous wire strand that is laced back and forth between two concentric series of through-holes that are accurately drilled around the periphery of the frame.
Another technique for manufacturing such a gate is described in U.S. Pat. No. 5,465,480 issued to Karl, et al. In this approach, the gating grid elements are produced from a thin metal foil by cutting or etching the foil to produce the grid structure. The gird elements are connected to side electrodes in a desired pattern to produce the two sets of wires. The foil grid structure is made mechanically stable by attaching it to an insulating support member. After the then-rigid grid structure is affixed to the insulating support member, the grid elements are selectively severed from the side electrodes to form the interdigitated grid.
Yet another approach for manufacturing such a grid is described in the paper by Kimmel, J. R., et al., entitled “Novel Method for the Production of Finely Spaced Bradbury-Nielson Gates,” in Review of Scientific Instruments, Vol. 72, No. 12, December 2001, pp. 4354–4357. In this method, a guide is first manufactured out of a polymer block. The guide has a series of evenly spaced parallel grooves. A hole is drilled through the center of the polymer block; this hole eventually carries the ion beam. The machined polymer block is mounted on an insulated face of an H-shaped portion of a single sided, copper clad circuit board, with the grooves running from top to bottom of the H. The polymer-to-copper clad contacts are then fixed using an epoxy. Two small portions of the single sided copper clad board are fixed on the bottom side of the polymer in the region where the block extends over the center bar of the H-shaped copper frame.
A hand cranked, rotating screw is then used as a weaving instrument. In particular, a gold-plated tungsten wire runs from a spool over a directing screw and is coupled to the hand cranked screw by a belt. The loose end of the wire is then fixed such as by using an epoxy. A weight is hung from the wire between the directing screw and the spool in order to provide a constant tension on the wire.
Beginning at one side of the center hole, the hand crank is turned, which rotates the frame, drawing the thread from the spool. While watching through a microscope, an assembler feeds a first wire set through alternating grooves in the surface of the polymer and around the frame, making sure to touch both contacts on each pass. After winding the wire across the entire width of the opening, the wire is bound to both copper contacts on either side of the hole using an epoxy. A razor blade is then used to remove the segment of the wire between the two contacts on the side of the frame opposite the polymer.
Using the same procedure as for the first wire set, a second wire set is then wound through the grooves located between the wires of the first set. The ends of the wires are then cut, leaving wire only on the polymer side of the frame.